The present invention relates to an image processing apparatus and method that read color image data including a plurality of pieces of color component data from a document. The invention also relates to an image forming apparatus that forms an image based on read color component data. In particular, the present invention relates to a technique for preventing show-through of a document.
When an image processing device mounted on a scanner, a copier, a facsimile, or a multifunction peripheral reads an image contained on only the front side of a document, and the document has images on both sides thereof, the occurrence of so-called “show-through” has been a problem. Show-through is caused when the image on the back side of the document appears in the read image. Typically, the density of the entire image of the document is reduced to prevent show-through. However, this creates the problem that the density of not only the show-through portion, but also of the original image of the document is reduced.
A technique has been proposed as a solution to this problem. The technique: first calculates a density histogram two-dimensionally representing a relationship between density and frequency; identifies a density with the highest frequency as a density of a background color; converts densities around the density of the background color uniformly to the same level as the density of the background color; and thereby prevents the occurrence of show-through.
FIG. 7 is an example of a density histogram that represents a relationship between density and frequency. The density histogram is obtained by calculating the frequency of density occurrence for each predetermined density range in image data. In the example of FIG. 7 the densities ranging from 0 to 15 occur most frequently.
However, in the proposed technique described above, even if there are a plurality of colors that occupy large areas of the image data of the document, processing for preventing show-through is performed based on the density of only the color that appears with the highest frequency. As a result, show-through cannot be prevented in other parts.
When the color image of a document is read, it is possible to calculate density histograms for respective pieces of color component data, identify a density with the highest frequency in each of the density histograms as a density in a background color, and convert each of the color component data to prevent show-through. However, since converting each color component data is performed over the entire image data, color reproducibility of the entire image data will be degraded. For example, assume that for color image data including RGB color component data, an R color component density close to a density of an R color component of a background color is converted to be the same as the density of the R color component of the background color. In this case, a density of not only an R color component of a show-through portion, but also that of an R color component included in an original image of a front side of a document is converted so as to be the same as the density of the R color component of the background color. As a result, since the R color component in the original image of the document is enhanced or suppressed, the color of the entire image is altered.
An alternative technique may be to identify, from a plurality of colors represented by a plurality of pieces of color component data, a high-frequency color that appears in the image of a document with a frequency that satisfies a predetermined high-frequency condition; generate, for each high-frequency color, a set of individual-density conversion tables to convert the respective pieces color component data such that a color close to the high-frequency color becomes substantially the same as the high-frequency color; and convert, for color image data representing the color close to the high-frequency color, the pieces of color component data based on the set of individual-density conversion tables corresponding to the high-frequency color. Thus, even if there are a plurality of high-frequency colors in the image of the document, it is possible to prevent show-through in the portions that correspond to the high-frequency colors. Additionally, since conversion of each color component data is only performed for color image data representing high-frequency colors, color reproducibility of the entire image is not degraded.
However, if there are dots (halftone dots) in a document to be read, the dot and background portion where there are no dots are divided into different color distributions in color image data read from the document. Therefore, since the color of the dot portion is not identified as a high-frequency color, one show-through that occurs in the dot portion may not be able to be removed.